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					            Optimization & Revitalization of Existing Reciprocating Compression Assets

                                            W. Norman Shade, PE
                                              ACI Services Inc.



Many reciprocating compressors in North America and throughout the world have been in service for decades,
some operating for more than half a century. Most older models are no longer optimal, and many are not
dependable for current process requirements. Over time, technology has improved and energy costs have
increased, making it economically attractive to optimize and revitalize existing compression assets with
components that take advantage of modern technology. Efficiency, reliability and maintainability can be greatly
improved through custom engineered solutions. Changes may range from conversion to better compressor
valves, to the addition of properly designed automatic unloading devices for better control, to the installation of
new purpose-built compressor cylinders and components, to the complete reapplication of an entire compression
system to a new service.

Even relatively new compressors can become inefficient and mis-matched as production needs change. In other
cases, from the time the units were installed, actual operating conditions never quite matched the design
conditions specified when the equipment was ordered. In still others, manufacturers may have missed the mark
when designing and applying their products, so that the equipment underperforms or is unreliable for its intended
service.

Whatever the reason, changing valves or cylinders, adding unloaders and automatic control, or reapplying,
existing reciprocating compressors is frequently necessary and justifiable. In the case of newer compressors, the
OEMs may have standard cylinders that can be retrofitted to their frames, however that usually requires
expensive changes to the process piping, pulsation vessels, and cylinder mounting. In the case of mature
compressor models that are no longer in production, new OEM cylinders tend to be expensive and have long lead
times, if available at all. In some applications, when new compressors are selected or used compressors are
redeployed, the standard OEM cylinder line-up may not include the optimal cylinder bore size or working pressure
rating for the desired operating conditions. Any of these situations are candidates for custom engineered cylinders
and unloading devices that provide cost-effective solutions for preserving the value and reducing the operating
cost of existing compression assets.


NEW CUSTOM ENGINEERED REPLACEMENT CYLINDERS

With over 30 years of designing and manufacturing custom engineered compressor cylinders, internal
components, valves, and unloading systems, the author’s company has helped a large number of compressor
users improve their competitive edge by providing creative solutions that improve their existing compression
assets to improve performance, efficiency, reliability, maintenance cost, and safety. These efforts have provided
solutions to operating problems, have improved the performance and have increased the reliability of
reciprocating compressors. The extensive experience in designing and manufacturing custom engineered
compressor cylinders is represented in Figures 1 and 2 below. Custom engineered cylinders have been applied
on all major brands of compressors ranging from small 3.0 in. (76.2 mm) stroke, 1800 rpm high-speed models to
giant 20.0 inch (508.0 mm) stroke, 330 rpm frames, with bore sizes ranging from 1.33 to 36.0 in. (33.8 to 914.4
mm), and working pressures from as low as 50 psig (3.5 bar) to as high as 12,000 psig (827.4 bar) for upstream,
gas transmission, gas storage, chemical process, refinery, high-pressure air and other services.


New compressor cylinders can be custom engineered and purpose built to meet customer specifications or to
solve problems with existing OEM designs. In most cases, the cylinders can be “bolt-in” replacements to match
existing process piping connections and mounting locations, which avoids expensive system piping and
foundation modifications. Cylinders can be jacketed (water cooled) or non-jacketed (air cooled) and made from
appropriate material for the application. Ductile iron castings are most commonly used in recent years, although
gray iron, steel or stainless castings are also used, as are steel and stainless steel forgings. Reduced hardness
materials are available for sour gas and other applications requiring NACE specifications. Cylinder bores may be
supplied in the virgin material condition, ion nitride hardened for improved wear resistance, or lined with a
replaceable sleeve. Depending on the requirements, existing internal components may be reusable, however new
CO 2009 Optimisation and Troubleshooting of Compressors and Turbines             Aberdeen, U.K. 3-4 February 2009
valves, piston rod packing and maintenance friendly piston and rod assemblies are often included. A wide range
of manual and automatically controlled clearance volume pockets and end deactivators are also available, custom
engineered for the specific application requirements.
                       40                                                                                              100,000
                                   ACI Services, Inc.                                                                                                                ACI Services, Inc.
                       35       Custom Cylinder Designs                                                                                                           Custom Cylinder Designs
                                 Bore (in.) v. Stroke (in.)                                                                                                       MWP (psig) v. Bore Dia. (in.)
                       30                                                                                               10,000




                                                                                                 Cylinder MWP (psig)
 Bore Diameter (in.)




                       25

                       20                                                                                                1,000

                       15

                       10                                                                                                 100

                        5

                        0                                                                                                  10
                            0      2     4     6    8    10     12     14   16   18   20   22                                    0   2   4   6   8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
                                                        Stroke (in.)                                                                                       Bore Diameter (in.)

Fig. 1: Custom cylinder experience – bore dia. vs. stroke                                       Fig. 2: Custom cylinder experience – pressure rating vs. bore diameter


Case History 1: Increased Pipeline Compressor Capacity. In the first example, a mid-continent USA natural
gas pipeline operator had several existing Clark HBA and HRA integral engine compressors with cylinders that no
longer served the operating needs of the compressor station. The customer specifications dictated an
approximate 32% increase in bore size in a new cylinder that provided maximum operating efficiency. In
addition, the cylinder was to have external dimensions that permitted the use of the existing piping system,
cylinder supports and foundation. It was readily apparent that a conventional cylinder design would never meet
all of the requirements of the specification. The OEM, who supplied the engines, agreed, but offered a
conventional design that did not meet the specifications. The Four-PosterTM cylinder offering was sufficiently
unique that the author’s company received a patent on the new design.

                                                                                                                       In addition to meeting all the specification
                                                                                                                       requirements, the cylinders were offered at a lower
                                                                                                                       price and shorter delivery than offered by the OEM.
                                                                                                                       In this design, all valves were accessible from the
                                                                                                                       top of the cylinder and the front and rear head,
                                                                                                                       piston assembly, liner and packing case were
                                                                                                                       accessible from the front of the cylinder. A total of
                                                                                                                       42 new 12.50 inch (317.5 mm) bore, 1000 psig (69.0
                                                                                                                       bar) cylinders were retrofitted onto multiple 17.0 in.
                                                                                                                       (431.8 mm) stroke compressors. Closed loop tests
                                                                                                                       certified the efficiency of the cylinders. Significant
                                                                                                                       savings resulted from the re-use of the piping and
                                                                                                                       cylinder supports and the absence of foundation
                                                                                                                       changes. Subsequent to the supply of these 42
                                                                                                                       cylinders, other orders were received for four more
                                                                                                                       identical cylinders and sixteen cylinders with slight
                                                                                                                       modifications for another OEM’s 14.0 in. (355.6 mm)
                                                                                                                       stroke engines. All cylinders delivered trouble free
Fig. 3: Retrofitting custom engineered cylinders increased bore size by                                                service.
32% on this 17 in. (431.8 mm) stroke Clark integral compressor.


Case History 2: Improved Service Life. A USA Gulf Coast chemical plant had a Worthington BDC off gas
compressor with cast iron cylinders that were constantly under chemical attack from extremely corrosive gas.
Cylinder life was typically only 3 to 5 years, and the OEM had not resolved the problem to the satisfaction of the
customer, who required a functionally equivalent cylinder design that would directly replace the existing cylinder
to the extent that all other parts of the assembly could be re-used with the new cylinder body.




CO 2009 Optimisation and Troubleshooting of Compressors and Turbines                                                                                    Aberdeen, U.K. 3-4 February 2009
The customer had attempted to produce replacement
cylinders of the same geometry in a ni-resist material,
however, the cylinder geometry and jacketed design
were not compatible with ni-resist foundry practices
and requirements. As shown in Figure 4, the author’s
company redesigned the exterior of the jacketed, 400
psig (27.6 bar), 12.0 in. (304.8 mm) bore, 9.5 in.
(241.3 mm) stroke cylinder body to simplify and
enhance foundry procedures. Proper core support
and clean out and pattern orientation in the mold
resulted in sound castings when produced in ASTM
A316 series stainless steel.

The change to A316 stainless steel resolved the
corrosion problems and eliminated the periodic safety
inspections that had been required with the original
cast iron cylinders. For more than 20 years, the
replacement cylinders have been more productive,
and maintenance costs have been reduced. Similar
                                                          Fig. 4: This custom engineered ASTM A316 cast stainless steel
replacements of this type were subsequently provided      cylinder eliminated a severe corrosion problem with the cast iron
for 19.5 and 7.75 in. bore (495.3 and 196.9 mm)           cylinder that it replaced.
cylinders for adjacent stages.

Case History 3: Providing Safe and Reliable Production Operation. A large Cooper Bessemer V250 integral
engine compressor in an Austral-Asian fertilizer production plant had experienced repeat failures of the 3rd stage
                                       5


syn gas cylinder. The 6.50 in.
(190.5 mm) lined bore diameter, 20
in. (508.0 mm), 6000 psig (413.7
bar) forged steel cylinder was a
design that employed transverse
tie-bolts for reinforcement. After
several years of service, cracks
would originate in the discharge
valve pockets and migrate to the
tie-bolt clearance holes as shown in
Figure 5. A thorough engineering
analysis     revealed    very   high
stresses and unacceptable fatigue
safety factors in the valve seats as
shown in Figure 7.

Since the high-pressure 3rd stage
was normally operating at 93% of
rated rod load, and the 1st and 2nd
stages were lower at 77% and 59%,
respectively, a performance study
was completed to determine an
optimal bore size for the new 3rd
stage cylinder. Interacting with the
plant engineering staff, it was                 rd
                                       Fig. 5: 3 stage syn gas cylinder showing location of leak detection.
agreed that the new cylinder bore
diameter could be reduced to 6.00 in.(152.4 mm), which cut the normal normal operating rod load to 84% of rated,
while still maintaining acceptable rod loads of 79% and 68%, respectively, on the 1st and 2nd syn gas stages. It
was also determined that the low molecular weight permitted a reduction in valve diameter, which significantly
reduced the preload that on the valve cap bolting, while also increasing the section thickness in the critical
corners where the valve pockets intersected the main bore.

A new 6.00 in. (152.4 mm) AISI 4140 forged steel bolt-in replacement cylinder was designed and extensively
analyzed to achieve acceptable fatigue safety factors. The new jacketed cylinder, shown in Figure 6, was fully

CO 2009 Optimisation and Troubleshooting of Compressors and Turbines                 Aberdeen, U.K. 3-4 February 2009
                                                            Fig. 7: Finite element stress analysis of high-pressure cylinder body showing
                                                            high stresses in valve seat fillets.

                                                            machined from a solid block forging that was 3 in. (76 mm)
                                                            thicker than the original cylinder. The problematic tie- bolts
                                                            were eliminated, and the critical valve pocket fillet geometry
                                                            was carefully controlled and then shot peened after machining
                                                            to provide beneficial compressive preload stresses. Water
Fig. 6: New AISI 4140 forged steel cylinder with water      jacket side covers were fully machined from aluminum plate
jacket, weighing more than 10,000 lb. (4335 kg).            material to eliminate a corrosion problem that was prevalent
                                                            with the original cast iron side cover design.

Case History 4: Eliminating Extremely Serious Piston Failures. A major oil company was experiencing
frequent and rapid piston failures on the 2nd stage of a reapplied 4000 hp (2983 kw) Superior W76 compressor
used for sour gas lift service on an offshore platform. Not only was
downtime causing six figure daily production losses, operating
personnel were fearful that the piston failures could lead to a
breach of the lethal sour gas, which would be extremely dangerous
on an offshore platform. The OEM and another compressor service
company had both tried to solve the problem with new pistons, but
failures continued in as little as 8 days of operating time. Upon
being consulted for help, the author’s company conducted a
thorough assessment and found that very high 2nd stage operating
temperature, coupled with rod loads that were at or above the
manufacturer’s rating, were the root causes of the rapid,
catastrophic failures of the 18.0 in. (457.2 mm), 7.0 in. (177.8 mm)
                                                                                    Fig. 8: Multi-piece Piston finite element stress plot.




                                          nd                                                                                          nd
Fig. 9: Original 18.0 in. (457.2 mm) OEM 2 stage cylinder [left] that experienced   Fig. 10: New 18.125 in. (460.4 mm) bolt-in 2           stage
frequent rapid, catastrophic piston failures due to a mis-application..             replacement cylinder.

CO 2009 Optimisation and Troubleshooting of Compressors and Turbines                               Aberdeen, U.K. 3-4 February 2009
stroke multi-piece aluminum and ductile iron pistons. As shown in Figure 9, the cylinder was an unusual shape
that resulted in a very short piston length relative to the diameter. A finite element stress analysis, as shown in
Figure 8, showed that the original piston design had a fatigue safety factor of less than 1.0 at the required
operating conditions, which explained the rapid failures. Unfortunately, the alternatives for resolving the problem
were very limited.

The short cylinder and OEM limits on the
permissible reciprocating weight precluded the
use of a stronger, less temperature sensitive
ferrous material piston. Adding interstage
cooling to reduce the 194°F (90°C) suction
temperature was not practical on the offshore
platform due to space limitations. The OEM did
not have an alternative cylinder that would
solve the problem, and lead time for a
complete new replacement compressor
package was about 18 months at the time. A
custom engineered replacement cylinder was
determined to be the preferred solution, but
lead time was also prohibitively long.
Concurrent design and analysis efforts
established that a one-piece solid aluminum
piston, although not having infinite life, would
operate for a longer and reasonably predictable
period of time. This was adopted as a
temporary measure until a new replacement
cylinder could be designed and manufactured. Fig. 11: New 21.0 in. *533.4 mm) bolt-in 1st stage replacement cylinders with
With the one-piece pistons providing additional manual variable volume clearance pockets.
time for development of a permanent solution,
further review of the operating conditions of the 1st and 2nd stages showed that further benefits were achievable by
changing the two 1st stage cylinders along with the 2nd stage. By changing the two 1st stage cylinders from 20.0 in.
(508.0 mm) to 21.0 in. (533.4 mm) and the 2nd stage from 18.0 in. (457.2 mm) to 18.125 in. (460.4 mm), a better
balance of rod loads and interstage temperatures was achieved, while providing the compressor with an
increased capacity rating that was an unexpected bonus for the operator.

All three of the new ASTM A395 ductile iron cylinders were designed longer to accommodate longer ductile iron
pistons that were capable of the required loading. In order to do this while also maintaining process piping
connection points, special cast ductile iron offset adapters were designed and manufactured for the suction and
discharge connections. The pistons were high-performance, two-piece ductile iron castings with internal ribs. Both
the cylinders and the pistons were extensively analyzed with finite element stress models. Standard Ariel variable
volume clearance pockets were adapted to the new 1st stage cylinders. The new cylinders received hydrostatic
tests at 1.5 times working pressure, followed by helium submersion tests at working pressure. Field-installed on
the original frame as shown in Figures 10 and 11, the new cylinders have completely eliminating the piston
failures, while providing the production platform with several percent more capacity than was achievable with the
original OEM cylinder configuration.

Case History 5: Increasing Compressor Efficiency and Throughput: A single-stage, gas engine driven,
Dresser Rand 6HOS-6 compressor was applied in natural gas pipeline service. The operating conditions of the
pipeline were such that the engine driver was fully loaded making it the limiting factor for throughput of the com-
pressor station. Performance tests of the original OEM cylinders, which were misapplied, showed that valve
losses were very high for the low ratio application because of insuf-ficient valve sizes and gas passages in the
OEM cylinders being used on the frame. New 10.5 in. (266.7 mm) bore, 6.0 in. (152.4 mm) stroke, 1200 psig
(82.7 bar) bolt-in replacement cylinder assemblies were designed with larger valves and generous internal gas
passages. As shown in Figure 12, the new cylinders were installed in place of the original cylinders in less than
two days, maintaining all existing process piping connections and mounting. Performance tests of the new
cylinders showed an efficiency gain of about 10%, bettering the guarantee of 8% and leading to a corresponding
10% throughput gain for the compressor station.



CO 2009 Optimisation and Troubleshooting of Compressors and Turbines                  Aberdeen, U.K. 3-4 February 2009
Fig. 12: The first of six new high efficiency bolt-in replacement cylinders [left] is installed on a six-throw, single-stage D-R 6HOS-6 frame. Two
of the original OEM cylinders are shown on adjacent throws [right] prior to replacement.


Case History 6: Increasing Cylinder Working Pressure. A USA natural gas storage facility had a requirement
for increased operating pressure. The compression was provided by four Cooper Bessemer GMVA-10 integral
engine compressors each having four older style vertically-split valve-in-head cylinders. The OEM, as well as the
author’s company, was asked to re-rate the original cylinders; however with large gasketed joints as shown in
Figure 13, the cylinders were only marginally adequate for their original pressure rating and therefore incapable of
further uprating. The OEM offered new higher-pressure cylinders, however, they would require major
modifications to the ASME coded pulsation bottles and process piping, greatly increasing the cost and the install-




Fig. 13: Original OEM 1100 psig (75.8 bar) valve-in-head type        Fig. 14: New custom engineered 1200 psig (82.7 bar) bolt-in replacement
cylinders.                                                           valve-in-barrel type cylinders.

ation time for the project. The author’s company measured critical interface dimensions on site without removing
the old cylinders and then designed new 12.0 in. (304.8 mm) bore, 14.0 in. (355.6 mm) stroke, 1200 psig (82.7
bar) ASTM A395 cast ductile iron bolt-in replacement cylinders as shown in Figure 14. The design was able to
accommodate the use of the original double deck valves, packing cases and automatic fixed volume clearance
pockets. As the new cylinders were completed, the original valves, packing, and pockets were reconditioned, and
the valve pocket volume bottles were re-rated for the higher working pressure. The field conversion went
smoothly and the customer’s facility was available, on schedule, to handle the higher operating pressure.

CO 2009 Optimisation and Troubleshooting of Compressors and Turbines                                    Aberdeen, U.K. 3-4 February 2009
Case History 7: Improving Safety and Maintainability. Another USA natural gas storage facility required
increased operating pressure. The facility had five 1940s vintage Cooper Bessemer GMV compressors with
valve-in-head cylinders. As shown in Figure 15, the original cylinders had two separate suction and two separate
discharge connections, and the cylinders were vertically split so that the outboard suction and discharge flange
connections had to be disturbed whenever it was necessary to access the piston, rod or packing for maintenance.
Although the OEM indicated that a modest pressure re-rate of the cylinders might be possible, the station opera-




Fig. 15: Original problematic valve-in-head cylinders required     Fig. 16: New bolt-in replacement valve-in-barrel cylinders reused
disturbing the outboard piping connections to access the piston,   original valves, packing and cylinder unloaders. Piping connections
piston rod and packing.                                            are no longer disturbed for piston, rod and packing maintenance.

tors were concerned about the safety since the existing cylinders sometimes had gasket leakage problems at the
original pressure rating. They wanted to avoid any gas piping or pulsation bottle changes and they wanted higher
pressure cylinders that eliminated the problematic gaskets and the cumbersome and costly maintenance
procedures that required disturbing gas piping connections to access the cylinder bore. In order to meet this
challenge, a special valve-in-barrel cylinder design was developed, and 20 new 7.0 in. (177.8 mm) bore, 14.0 in.
(355.6 mm) stroke, 2000 psig (137.9 bar) ASTM A395 cast ductile iron, jacketed, cylinders were manufactured
and installed on the existing compressors as shown in Figure 16. Valves, packing and unloaders were
reconditioned and reused in the new cylinders.

Case History 8: Emergency Replacement of Obsolete Cylinders. A chemical plant had an emergency when a
critical Pennsylvania ATP off gas compressor unexpectedly developed internal process gas leaks into the cooling
water jacket. The OEM no longer had the original
cylinder body pattern, so their lead time for a new
cylinder approached one year. Since loss of this
compressor required shutting down the entire plant,
emergency temporary repairs were made to the
cylinder, and the author’s company was contacted to
provide a new cylinder. Critical interface measurements
were made on site, and a new design 20.5 in. (520.7
mm), 11.0 in. (279.4 mm) stroke, 150 psig (10.3 bar)
cylinder was expedited from ASTM A395 cast ductile
iron to strengthen the weak, corrosion sensitive
sections. The new cylinder was delivered in only 20
weeks, including the time for design, pattern
construction, casting, machining, hydrostatic testing
and installation of a new ni-resist liner (Figures 17 - 18).
                                                                    Fig. 17: Liner installation.          Fig. 18: Finished cylinder.

Case History 9: Extending OEM Standard Cylinder Offerings. An upstream production company had a
relatively new Ariel JGJ/6 compressor used for acid gas injection to stimulate oil production. The 600 hp (447 kw),
1500 rpm compressor was originally configured as a 5 stage unit. Once the field was operating, however, it
became evident that production could be enhanced by increasing the injection pressure to a level that exceeded
the capability of the compressor as originally configured. It was determined that a 6th compressor stage was
required, but the OEM did not have a suitable high-pressure cylinder for that particular frame.

CO 2009 Optimisation and Troubleshooting of Compressors and Turbines                               Aberdeen, U.K. 3-4 February 2009
In addition, the aggressive nature of the acid
gas required the use of material produced to
NACE specifications; and the limited physical
size constraints required a higher material
strength than could be achieved with the use
of reduced hardness alloy steels. 17-4PH
stainless steel was therefore the material of
choice for the new cylinder. With the approval
of the OEM, the author’s company designed
and manufactured a special 2.75 in. (69.9 mm)
bore, 3.5 in. (88.9 mm) stroke, 3000 psig Fig. 19: Special 3000 psig (206.8 bar) 17-4PH stainless steel tail rod cylinder
(206.8 bar) cylinder. The tail rod design, which designed for acid gas injection service.
is shown in Figure 19, was completely machined from a solid 17-4PH double H1150 heat treated stainless steel
forging. This cylinder was added to the original compressor as a 6th stage to increase final injection pressure.

Case History 10: Filling gaps in OEM Standard Cylinder Offerings. A distributor packager of new Gardner
Denver and remanufactured Joy 7.0 in. (177.8 mm) stroke high-pressure air compressors used in portable diesel
engine driven compressor packages for the rotary drilling industry was encountering application conditions for
which the OEM cylinders were not optimally suited. Given the maturity of the compressor product line, the OEM
was not very interested in modifying the existing cylinder designs or creating new cylinder designs.

The author’s company reviewed the
range of required application conditions,
measured       the   mounting     interface
dimensions, and subsequently developed
a family of new 7.0 in. (177.8 mm)
cylinders that fit the compact vee-type
Joy and Gardner Denver compressor
frames and that were optimally sized for
the air drilling application requirements.
The bore sizes of the ASTM A395 cast
ductile iron, jacketed cylinders, ranged
from 3.5 to 4.5 in. (88.9 to 114.3 mm)
with a working pressure of 2500 psig
(172.4 bar). Cylinders are now supplied
on a regular basis to meet this distributor
packager’s needs, and additional bore
sizes are being considered to further
expand the range of pressures and flow
rates offered to meet the needs of the        Fig. 20: 2500 psig (172.4 bar) cylinder family developed to fill a gap in the OEM’s
drilling market as shown in Figure 20.        offerings for high-pressure air drilling service.


Many more examples can be provided, however, the forgoing list shows the various kinds of applications for
which new custom engineered, purpose built reciprocating compressor cylinders can be attractive alternatives for
optimizing and revitalizing existing compression assets.


RE-APPLIED COMPRESSOR CYLINDERS

Although new, custom engineered and purpose-built cylinders can provide optimal performance and reliability,
there are frequent examples where surplus or previously decommissioned cylinders can be reconditioned and re-
applied onto existing compressor frames. New OEM cylinders can also be installed on different OEM frames that
have similar strokes and rod diameters. Of course, diligent and experienced engineering design is required to
match the different mounting, bolting, piston rod and, if the stroke is different, the piston. Consideration must also
be given to matching reciprocating weights with the compressor frame’s capability. Maximum pressure ratings,
Internal gas and inertia rod load ratings, and pin reversal requirements must be considered in the reapplication as
well. Done properly, reapplication of cylinders is a reliable alternative that can often reduce the capital cost and
will almost always reduce the cylinder lead time. Several examples demonstrate the potential of this engineered
approach.

CO 2009 Optimisation and Troubleshooting of Compressors and Turbines                        Aberdeen, U.K. 3-4 February 2009
Case History 11: Adapting Remanufactured Cylinders to An Existing Compressor. A natural gas pipeline
transmission station had two 1950s vintage I-R KVG 10/3 integral engine compressors in service. Although
having a provision for three compressor throws, only two of the throws were previously equipped with cylinders,
the center throw being unused for more than 50 years. Conditions had changed over that period of time so that
                                                    the 600 hp (447 kw), 330 rpm compressors were no longer
                                                    fully loaded. The pipeline operator had determined that, by
                                                    adding another identical cylinder to the inactive throw in
                                                    parallel with the existing cylinders in a single-stage
                                                    configuration, the engine could be fully loaded to increase the
                                                    station’s capacity. Inquiries were made to the OEM and
                                                    several other companies. The OEM still had the original
                                                    pattern and was able to quote new identical cylinders,
                                                    however lead time for that option was about 40 weeks. The
                                                    author’s company also offered to design and manufacture
                                                    new bolt-in replacement cylinders, however, the best possible
                                                    lead time was 26 weeks. Neither of these alternatives were
                                                    adequate for the operator’s requirement to have the
                                                    conversion completed so that both compressors would be in
Fig. 21: One of two used I-R cylinders prior to the
remanufacturing process.
                                                    service within no more than 20 weeks after the release of the
                                                    order. An extensive search for identical used or surplus cylin-




Fig. 22: Performance model for 3-cylinder, single-stage I-R         Fig. 23: Remanufactured cylinders with new fixed volume clearance
compressor showing all load steps required for system flexibility   pocket heads and valve caps undergoing an 8-hr factory hydrostatic test
without overloading at the maximum specified discharge pressure.

ders produced no results either, although the author’s company was able to find two similar 15.0 in. (381 mm)
stroke I-R process cylinders, Figure 21, that had the same flanges and distance between flanges. It was
determined that with modifications to the combination crank end head and distance piece, the process cylinders
could be adapted to fit the existing KVG compressors. A performance model, Figure 22, showed that the cylinders
would meet the required operating conditions if all cylinders were modified by lining to a 10.16 in. (258.1 mm)
bore diameter.

Having no time to lose, two used cylinders were purchased and taken to the factory for cleaning, inspection and
hydrostatic testing to confirm that the cylinder bodies were sound. The cylinders were then modified to fully
restore all fit dimensions, sealing surfaces and threads to ensure their integrity for the reapplication. All new
bolting, seals and gaskets were installed. After machining, a second hydrostatic test of the remanufactured
cylinders was conducted at 1.5 times maximum working pressure, as shown in Figure 23. Concurrent with the
completion of the “new” remanufactured cylinders, new liners, non-lube piston and rod assemblies, double deck
poppet valves, valve cages, valve caps, outer end automatic fixed volume clearance pocket heads, and highly
efficient radial poppet suction valve deactivators were designed and manufactured for the four existing cylinders
on the two compressors as well as for the two remanufactured cylinders. A complete API 618 design approach 2
pulsation study was also conducted on the entire system with the new configuration. This study revealed high
pressure drops in the existing plant suction pulsation control arrangement with the higher flow, three cylinder
configuration, and mitigating actions were quickly identified and added to the conversion plan.


CO 2009 Optimisation and Troubleshooting of Compressors and Turbines                              Aberdeen, U.K. 3-4 February 2009
When all components and cylinder
assemblies were completed, the station
was taken off line for the conversion. The
suction and discharge bottles were
removed and taken to an ASME code
welding shop for addition of the
incremental connections, followed by
recertification.  Some     simple    piping
modifications were also made to mitigate
the pressure drop problem that had been
revealed by the pulsation study. Using
special field machining equipment, the
existing liners were machined out of the
existing cylinders without removing them
from the compressor. New liners and the
other new components were then installed
along with the incremental remanufactured
cylinders. The revised bottles were
reinstalled and the system was completed
and commissioned on schedule. One of
the completed units is shown in Figure 24.              Fig. 24: One of two reconfigured KVG units after adding a 10.16 in. (258.1 mm)
                                                        diameter remanufactured cylinder [center] and adding new clearance pockets, radial
At this time the compressors have been in               poppet suction valve deactivators, and other new components to all the cylinders.
service for over 3 years.

Case History 12: Mixing Used OEM Cylinders and Frames to Optimize Performance. A natural gas
processing plant wanted to reapply an existing 2000 hp (1491 kw), 900 rpm gas engine and compressor package
for a propane refrigeration process. Process conditions were fixed, so that cylinders had to be carefully selected
to match the operating requirements. Using the preferred 6.0 in. (152 mm) stroke Superior WH64 compressor
frame, initial performance sizing resulted in a selection of 25.5 in. (648 mm) diameter Superior cylinders for both
the 1st and 2nd stages, however the 2nd stage cylinders would have had to operate very close to their working
pressure limit. The same concern existed for the 20.0 in. (508 mm) 3rd stage cylinder. The OEM was not
interested in producing special cylinders for a single unit, and no used cylinders with an adequate working
pressure could be found.




Fig. 25: Adaptation of 5.5 in. (140 mm) stroke Ariel cylinders to a   Fig. 26: 2000 hp (1491 kw), 900 rpm, gas engine driven 3-stage
Superior 6.0 in. (152 mm) stroke Superior compressor frame.           propane refrigeration compressor.


As a result, a new 6.5 in. (165 mm) stroke Ariel JGC/4 compressor was considered, but this resulted in concern
about the reliability of the valves with the heavy propane gas. Switching to a 5.5 in. (140 mm) stroke Ariel JGD/4
compressor made the valve reliability more promising, but it could not produce the required flow with the available
standard Ariel cylinders. It was finally determined that the solution could be optimized by adapting Ariel JGD
cylinders to fit on the Superior WH64 compressor frame. Ariel 22.0 in. (559 mm) diameter JGD cylinders were
selected for the 1st stage and the two 2nd stage cylinders, and an Ariel 19.625 in. (499 mm) diameter JGD cylinder
was selected for the 3rd stage. Used Ariel cylinders were found that could be reconditioned for all but one of the
CO 2009 Optimisation and Troubleshooting of Compressors and Turbines                              Aberdeen, U.K. 3-4 February 2009
four that were required, however since the JGD cylinders are designed for a 5.5 in. (140 mm) stroke, their use on
the Superior frame required redesigning the piston and piston rod to accommodate the longer stroke. In addition,
special mounting adapters were required to match the different bolt patterns on the cylinders and the frame, as
shown in Figure 25. The author’s company provided the special engineered components to accomplish this
adaptation. At this time, the unit, shown in Figure 26, has been in service for approximately 3 years.

Case History 13: Extending Obsolete Compressor Life with New OEM Cylinders. A natural gas production
company had an obsolete 6.0 in. (152.4 mm) stroke Knight KOCA-2 compressor frame with fabricated (welded)




Fig. 27: Obsolete KOCA/2 compressor prior to conversion.             Fig. 28: Loading map for new 2-stage configuration.

steel cylinders, as shown in Figure 27. The cylinders
were susceptible to cracking problems in service and
were no longer safely repairable. The original
compressor OEM had been out of business for more
than two decades, so neither new nor repairable
used cylinders were realistic alternatives. Lead time
for new compressor packages was close to 52 weeks
and production losses would be substantial if the
compressor could not be returned to service in a
much shorted time. The author’s company was asked
to evaluate alternatives. Although new custom
engineered cylinders were an option, lead time was
still somewhat longer than desirable, considering the
cost of lost production. A much shorter lead time was
possible by adapting new Ariel JGK cylinders to the
KOCA frame. As long as changes were going to have
to be made to the cylinders, bottles and piping Fig. 29: Obsolete KOCA/2 frame with new Ariel cylinders applied.
anyway, the producer decided that it would be desira-
ble to optimize the compression system to accommodate an anticipated suction pressure decline over time, while
still utilizing as much of the gas engine driver power as practical to deliver flow. This ultimately required converting
from a single-stage to a 2-stage configuration with 8.375 in. (212.7 mm) and 6.25 in. (158.7 mm) bore cylinders,
which provided optimal coverage of the operating conditions and maximum driver power utilization as shown in
Figure 28. Even more importantly in this case, the solution could be implemented in less than 20 weeks time.

New 5.5 in. (139.7 mm) stroke Ariel cylinders were converted to 6.0 in. (152.4 mm) stroke with new custom-
engineered piston and rod assemblies, rod packing and wiper assemblies, and mounting adapters to match the
KOCA frame. New ASME coded pulsation bottles were also designed and manufactured, as shown in Figure 29,
and piping modifications were made to adapt to the original skid and utilize the process gas cooler. An API 618
Design Approach 2 pulsation study was conducted, and the system piping design was tuned to provide effective



CO 2009 Optimisation and Troubleshooting of Compressors and Turbines                   Aberdeen, U.K. 3-4 February 2009
pulsation control over the broad operating range. The unit was been in service for more than two years, and
additional units are now being considered for similar conversions.

Case History 14; Completely Reconfiguring an Existing Compressor. An existing 600 hp (447kw), 588 rpm,
4-throw, 3-stage, 7 in. (177.8 mm) stroke Joy WB14 compressor was originally configured as shown in Figure 30
to compress 1303 cfm (42.5 m3/min) of bone dry nitrogen. Requirements changed such that only 314 cfm (10.3
m3/min) was required. The cost of recyling 76% of the flow was not acceptable, but the cost of a complete new,
but smaller, machine was equally difficult to justify. The owner/operator wanted to reconfigure the existing
compressor at minimal cost to operate more efficiently at the lower flow rate. In order to evaluate possible
alternatives, a complete performance model was developed for the existing configuration having two16.0 in.
(406.4 mm), one 10.5 in. (266.7 mm) and one 7.0 in. (177.8 mm) cylinders. The model was then revised in an
iterative manner to evolve a solution that let replacing the two 1st stage cylinders with reconditioned used 8.0 in.
(203.2 mm) cylinders, moving the original 7.0 in. (177.8 mm) 3rd stage cylinder to the 2nd stage, and installing a
reconditioned used 4.0 in. (101.6 mm) cylinder on the 3rd stage. The bottles were modified with new flanges to
adapt to the smaller cylinders. Most of the existing piping was reused by adding adapter sections.




Fig. 30: Prior to reconfiguring, this oversized 3-stage Joy WB14   Fig. 31: The reconfigured unit was sized to meet the process
nitrogen compressor required 76% of the flow to be recycled.       requirements without significant recycled flow.

By reconfiguring the compressor with reconditioned used cylinders and reapplying one of the existing cylinders,
as shown in Figure 31, this solution provided a cost effective solution that saved significant operating cost and
preserved the original investment of the compressor frame, motor and the balance of the system.


COMPRESSOR OPTIMIZATION AND AUTOMATION WITH CUSTOMIZED UNLOADING DEVICES

In addition to hundreds of successful examples of custom designed new cylinders and countless innovative
adaptations and reapplications of new and used OEM cylinders, there are even more examples of compressor
optimization with automatically actuated devices, often called unloaders, that control the load and flow.

Case History 15: Automation of Propane Refrigeration Compressor. A natural gas processing plant used a
400 hp (298 kw) Ajax compressor with no automatic control. The 2-
stage, 2-throw unit served as the low-pressure stage of a multi-unit
propane refrigeration system. As ambient conditions changed, the
propane boil-off rate varied, necessitating manual adjustments of the
compressor’s clearance pockets. Since the unit was not attended
24/7, the operators kept the unit under-unloaded so that it could
handle upsets without shutting down. Unexpected shutdowns stopped
the liquid stripping process and led to serious problems downstream
from the gas plant.

The solution involved the combination of a discrete step unloader and
an infinitely variable unloader providing, in essence, infinite variability        Fig. 31: Fine unloading steps for a wide range of
over a broad range of operating conditions, as shown in Figure 31.                 operating conditions are provided by a combination
                                                                                   of fixed and variable unloaders.

CO 2009 Optimisation and Troubleshooting of Compressors and Turbines                         Aberdeen, U.K. 3-4 February 2009
                                                            This gave the plant maximum compressor loading (and
                                                            maximum plant refrigeration and liquid stripping capacity)
                                                            without the risk of unexpected shutdowns.

                                                            The compressor conversion involved installing a
                                                            hydraulically actuated head end variable volume clearance
                                                            pocket on the 1st stage cylinder along with a pneumatically
                                                            actuated fixed volume clearance pocket on the head end of
                                                            the 2nd stage cylinder as shown in Figure 32. The control
                                                            pressure for the hydraulically actuated variable volume
                                                            unloader was provided by the use of discharge pressure
                                                            acting on an external accumulator containing hydraulic fluid
                                                            separated by a bladder. This approach eliminated the need
                                                            for separate pumps and power requirements. The control
                                                            pressure for the fixed volume pocket was provide by plant
                                                            are pressure.

                                                            Control algorithms, that were derived from a robust
                                                            reciprocating compressor performance modeling program
                                                            developed by the author’s company, were programmed into
                                                            a PLC, which then automatically controlled the unit with the
                                                            two unloaders. The conversion required about two days of
Fig. 32: New automatic clearance pockets added to propane   installation time, and it has been operating continually for
refrigeration compressor.                                   more than 3 years.

Case History 16: Pipeline Compressor Automation. A pipeline operator required the reapplication of five large
Clark TCV-12 and TLA-6 19.0 in. (483 mm) stroke integral engine compressors. An increased range of pressure
ratios required a reduction of existing fixed clearance. The budget was tight and required reusing as much of the
                                                                                          existing hardware as possible.
                                                                                          The author’s company modeled
                                                                                          the compressor performance
                                                                                          and developed an optimum
                                                                                          unloading configuration. The
                                                                                          front and rear heads and liner
                                                                                          ports were reconfigured to
                                                                                          reduce the cylinder fixed
                                                                                          clearance. The large 14.0 in.
                                                                                          (355.6 mm) diameter cylinders
                                                                                          were lined down to 12.5 in.
                                                                                          (317.5 mm). The compressor
                                                                                          valves were redesigned to
                                                                                          maximize the lift area and
Fig. 33: One of five large 19.0 in. (483 mm) stroke integral engine compressors that were minimize fixed clearance. A
completely configured for automatic operation with a broad range of pressure ratios.      total of 116 fixed volume clear-
ance pocket unloaders having volumes of 150 in3, 300 in3, 600 in3, or1500 in3 (2.5, 4.9,9.8 or 24.6 L) provided
the flexibility necessary to meet the wide range of compression ratios associated with the new application.
Novel piston de-signs permitted the use of existing piston rods while pro-viding piston to rod attachment
features that resulted in easier and safer assembly techniques. The revamped units, shown in Figure 33,
provided the required performance, which was more flexible and reliable than the OEM’s offering.

Case History 17: Replacement of Variable Speed Engine with Synchronous Electric Drive. A natural gas
producer had a 2-throw, 2-stage Superior W-72 reciprocating compressor driven by a 1600 hp (1193 kw), 900
rpm gas engine that had become obsolete, was inefficient and was expensive to maintain. In addition, due to local
air quality restrictions, and electric motor drive was considered as a preferred replacement for the engine drive.
The primary mode of flow control was via speed control on the engine, which provided a high level of flexibility.
However the operator did not want the complexity or cost of a variable frequency electric motor drive. Use of a
constant speed, synchronous motor drive, then, appeared to be impractical for the application that had to have a
wide range of flow control capability. The author’s company was requested to propose unloading equipment that
would provide optimal flexibility at reasonable cost. After weighing various options, the preferred solution involved

CO 2009 Optimisation and Troubleshooting of Compressors and Turbines                   Aberdeen, U.K. 3-4 February 2009
simply replacing the outer heads of each compressor cylinder and adding plug-type suction valve deactivators to
the head ends of both cylinders. As shown in Figure 34, the new 1st stage head was configured with a custom
engineered design having 3 different clearance pockets of 73, 146 and 292 in3 (1.2, 2.4 and 4.8 L) respectively.
                                                 The 2nd stage head was configured with a custom engineered
                                                 design having 2 identical clearance pockets of 170 in3 (2.8 L).
                                                 Each pocket was controlled with its own integral pneumatic
                                                 actuator that was controlled by a PLC control panel mounted




                                                              Fig. 35: The multiple pocket unloaders, together with valve deactivators on
Fig. 34: Two-throw compressor speed with multiple clearance   the two head ends, provided more flexibility with a synchronous motor drive
pockets on each compressor cylinder head end.                 than the same compressor had with a 33% speed turn down.

on the compressor package. Together with the two sets of head end suction valve deactivators, this arrangement
resulted in 31 discrete load steps that provided the very fine control shown in Figure 35. As shown, the
arrangement actually provided about 36% more turndown capability than the original variable speed drive.

Case History 18: Automation of Field Gas Gathering Compressors. A U.S.A. Appalachian natural gas
producer was operating three 2-stage Ajax DPC-600 compressors. Suction pressure swings lead to the units
shutting down to prevent overloading, thus reducing each unit's run time and requiring personnel to travel out to
the remote units to restart them. The operator
therefore had been running the units conservatively
in order to accommodate the pressure swings
without shutting down, however that resulted in less
overall production.

An engineering study was commissioned to
evaluate the compressor configuration to identify the
best means for operating each unit via automatic
control and unloading to cover the entire operating
range. Goals were to keep the engines running
safely as field condition changed, and also to
increase capacity during normal operation to
minimize the number of units that had to be
operated at any time. Various alternatives were
evaluated including a single 685 in3 (11.2 L) fixed
                                                       Fig. 36: With only manual control, this 2-stage Ajax compressor was
volume pocket; parallel 192 and 383 in3 (3.1 and 6.3 either under-loaded or subject to frequent shutdowns to avoid overload.
L) fixed volume pockets; parallel 114, 228, and 342
in3 (1.9, 3.7 and 5.6 L) fixed volume pockets, and an automatic variable volume pocket. In order to evaluate the
alternative solutions, performance was compared at more than 1000 operating points within the operating range
noted in Figure 37. The highest flow possible at each point was plotted for each case at each condition and the
curves were overlayed for comparison. Over the range of 160 to 240 psig (11.0 to 15.5 bar) suction and 900 to
1000 psig (62.1 to 69.0 bar) discharge, the highest average flow was possible with the automatic variable volume
pocket, however, the 3-volume pocket (the 2nd from the top surface in Figure 37) provided average flow that was

CO 2009 Optimisation and Troubleshooting of Compressors and Turbines                              Aberdeen, U.K. 3-4 February 2009
                                                                                   within 1% of the variable pocket (top
                                                                                   surface). The 3-pocket heads, Figure
                                                                                   38, were also slightly lower in cost
                                                                                   and significantly easier to control.
                                                                                   This solution was also a more
                                                                                   practical alternative for a remote field
                                                                                   application with limited technical
                                                                                   support and maintenance available.




Fig. 37: Comparison of various Unloading alternatives over full operating range.   Fig. 38: 3-volume clearance pocket unloader.



SUMMARY

The list of examples can go on and on, but the foregoing case histories provide arrange of good examples.
Optimization and revitalization has been successful on compressors used in air drilling, nitrogen, air procession,
air separation, chemical processing, field gas gathering, gas boosting, gas lift (EOR), gas transmission, gas
injection and withdrawal, refinery, refrigeration and specialty applications. Virtually any size and model can be
considered for improvements, whether or not the compressors are currently supported by OEMs or are obsolete.
Experience includes Ajax, Alley, Ariel, Chicago Pneumatic, Clark, Cooper-Bessemer, Dresser-Rand, Delaval,
Enterprise, Gardner Denver, GE Gemini, Hurricane, Joy, Knight, Neuman Esser, Norwalk, Penn Process,
Superior, Thomassen, and Worthington.


ACKNOWLEDGEMENT

The author acknowledges the legacy provided by many decades of technical leadership, innovation and
dedication of respected industry experts William Hartwick, Richard Deminski, Edward Miller, and Richard Doup,
as well as the ongoing contributions of Chad Brahler, Dwayne Hickman, Charles Wiseman, Larry Burnett and
their capable teams.

REFERENCES

1. Brahler, C.D.; Hickman, D.A. & Shade, W.N., Performance Control of Reciprocating Compressors - Devices for
   Managing Load and Flow, Gas Machinery Conference, Albuquerque, NM, Oct. 7, 2008.
2. Brahler, C.D. & Schadler, H.C., One Compressor OEM May Not Have the Optimum Equipment Selection, Gas
   Machinery Conference, Dallas, TX, Oct. 1, 2007.
3. Harbert, P.; Hickman, D.; Mathai, G., & Miller, R., An Automation Method for Optimizing and Controlling A
   Reciprocating Compressor Using Load Step, Speed and Suction Pressure Control, Gas Machinery Conference,
   Dallas, TX, Oct. 2, 2007.
4. Shade, W.N., Custom Engineered High-Performance Cylinders Revitalize Existing Compressor Assets, GMC
   Today, Oct. 4, 2004.
5. Shade, W.N., DCP Midstream Upgrades LaGloria Gas Plant, COMPRESSORTechTwo, Oct. 2007.
6. Shade, W.N., Modern Replacements for Old Valve-In-Head Compressor Cylinders, GMC Today, Oct. 2, 2007.
7. Shade, W.N., Re-Engineering Extends the Utilization of Pipeline Compressor Assets, GMC Today, Oct. 3,
   2006.

CO 2009 Optimisation and Troubleshooting of Compressors and Turbines                      Aberdeen, U.K. 3-4 February 2009

				
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